Automatic volume control transistor circuit arrangement



Feb. 22, 1966 w. F. HEINE ETAL Re. 25,963

AUTOMATIC VOLUME CONTROL TRANSISTOR CIRCUIT ARRANGEMENT Original FiledOct. 9, 1959 Our H c m T m6 Z mp M A 1 0 C 0 0 a V a A 5 G m 6 E M Q m om 1 mp B i Auouo SIG/14L Mpur INVENTORS Warm/v0 F BY lawu M591 Z2? @mArro /vn's Con/rem.

United States Patent 25,963 AUTOMATIC VOLUME CONTROL TRANSISTOR CIRCUITARRANGEMENT Wolfgang F. Heine, Huntington Station, and Kalju Meri,

Elmhnrst, N.Y., assignors to Mohawk Business Machines Corporation,Brooklyn, N.Y., a corporation of Maryland Original No. 3,019,396, datedJan. 30, 1962, Ser. No. 845,407, Oct. 9, 1959. Application for reissueJan. 29, 1964, Ser. No. 437,340

14 Claims. (Cl. 330-28) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification: matterprinted in italics indicates the additions made by reissue.

The present arrangement relates generally to automatic volume control(AVC) or gain control (AGC) circuits, and more particularly to anautomatic volume control circuit for a transistor amplifier operating inthe audio frequency range.

It is common practice to provide radio receivers with an AVC system tomaintain the carrier voltage at the detector at a substantially constantlevel. In receivers employing vacuum tubes, this is usually accomplishedby biasing the grids of the radio-frequency, intermediatefrequency andconverter tubes negatively with a direct voltage derived by rectifyingthe carrier signal. An increase in carrier signal will raise thenegative bias and thereby tend to counteract the increased signal byreducing the amplification. In this way variations in signal strengthdue to fading or on tuning from strong to weak carriers are smoothedout.

An AVC action is ordinarily obtained by deriving from a diode rectifierand filter a direct voltage proportional to the amplitude of the carrierat the diode input but free of the modulation component. The timeconstants of the filter circuit are adjusted so that the lowestmodulation frequencies do not reach the AVC output. At the same time,the time constants are small enough so that the rectified bias willfollow fairly rapid changes in carrier amplitude.

In vacuum tube circuits, the tubes controlled by the AVC system aregenerally of the variable-mu type in order to minimize the possibilityof cross-modulation. The recent advent of transistor circuits hascreated certain difiiculties in conjunction with AVC systems, fortransistors having characteristics equivalent to variable-mu tubes arenot available. It is therefore the conventional practice in AVCtransistor circuits to apply the direct-bias to the transistor so as toshift the operating point thereof toward the cutoff region. While thisarrangement is feasible if applied to transistorized high-frequencycarrier amplifiers, it leads to unbearable distortion when applied to anaudio-signal amplifier.

Accordingly, it is the principal object of this invention to provide anovel, distortion-free automatic-volume-control system for atransistorized amplifier.

More specifically, it is an object of the invention to provide an AVCsystem for a transistor amplifier operating in the audio range whichdoes not require an amplifying tube or transistor having curved orvariable-mu transfer characteristics and which does not entail changesin direct-current level.

Also an object of the invention is to provide an AVC transistoramplifier circuit of efficient and reliable design in which attack anddecay times may be readily adjusted, the arrangement generating nopopping sounds and introducing no distortion, unless severelyoverloaded.

Still another object of the invention is to provide in an AVC circuit atransistor device which acts as a variable impedance responsive to theamplitude of the signal,

Reissued Feb. 22, 1966 the impedance action being effected without adirect current applied to the collector of the transistor device.

For a better understanding of the invention as well as other objects andfurther features thereof, reference is made to the following detailedspecification to be read in conjunction with the annexed drawing wherebylike components in the several figures are identified by the referencenumerals.

In the drawing:

FIG. 1 is a schematic diagram of a preferred embodiment of theinvention;

FIG. 2 is a simplified form of the invention; and

FIG. 3 is the equivalent electrical circuit of FIG. 2.

Referring now to the drawing, and more particularly to FIG. 1, there isshown a transistorized audio-amplifier, including a first input stageprovided with a transistor 10, the output of this stage being fed tosucceeding stages of conventional design represented by the block 11. Itis to be understood that the invention is applicable to all known typesof transistors such as the N and P types, provided of course that theelectrodes are appropriately biased.

The input signal is applied at terminal 12 connected to the base B oftransistor 10, the base being coupled to ground through a resistor 13. Anegative bias is applied to the collector C of transistor 10 throughresistor 14. The output of the first stage is taken from the collector Cof transistor 10 and applied through coupling capacitor 15 to the inputof the succeeding stage 11. The emitter E of transistor 10 is connectedto ground through resistor 16.

In a conventional transistor amplifier circuit, resistor 16 would beby-passed for audio frequency currents by a condenser connectedthere-across. If the by-pass condenser is omitted, a substantial loss ingain will be experienced, the loss being in the order -30 db. This factis exploited in the present invention by providing a bypass condenser 17which is connected across resistor 16 through a variable controlimpedance constituted by a second transistor 18. Condenser 17 isconnected to the collector C of control transistor 18, the emitterthereof being grounded.

In accordance with the invention the impedance of control transistor 18is varied as a function of the volume level of the audio signal in theamplifier so as to compensate for changes in level and therebyautomatically to control the gain. This is accomplished by applying aportion of the output of the final amplifier stage 11 to the controltransistor 18 through a rectifier circuit constituted by diodes 21 and22 which are serially connected between ground and the base B oftransistor 18 through resistor 23 in series with resistor 24.

The signal from the output of the output stage is applied to therectifier circuit at the junction of diodes 21 and 22 through resistor25, and a negative bias for base B of the control transistor 18 relativeto emitter E is applied at the junction of resistor 23 and 24. Afiltering of the rectified signal is effected by means includingcapacitors 26 and 27 connected between ground and the opposing ends ofresistor 23.

The control transistor 18 will act as a variable impedance controllingthe gain in the amplifying transistor 10 without any direct-currentchanges in transistor 10 and without any transients in the signalamplified. The value of condenser 26 is chosen to obtain the desireddecay time and the value of resistor 25 is chosen to secure the desiredattack time.

It is to be noted, in connection with FIG. 2, that as the conductance ofcontrol transistor 18 is varied by the rectified and filtered signal, itacts only to adjust the degree of by-pass introduced by condenser 17 andit does not affect direct-current flow in transistor 10. This isequivalent to the action of variable resistor 18 in H6. 3, which servesto adjust the by-pass reactance path across resistor 16 without changingdirect current flow through the resistor.

Since the speed of automatic-veinmc-control (say 10 milliseconds) is inthe same order as the amplified frequencies in an audio amplifier, achange in direct current flow in resistor 16 as a result of theregulating action would appear as an unwanted signal in the audiooutput, the unwanted signal being heard as a popping sound. With thepresent invention such unwanted signals are obviated.

It is also significant that the transistor 18 acts as a variableimpedance device without there being a direct current voltage applied tothe collector C. The condenser 17 is interposed between the collector Cof transistor 18 and the direct current circuit.

While there has been shown what is considered to be a preferredembodiment of the invention, it is to be understood that many changesmay be made therein without departing from the essential scope of theinvention as defined by the claims.

What is claimed is:

1. An automatic volume control system for a transistor amplifierprovided with an amplifying transistor and a circuit therefor, inputmeans to apply an input signal to said transistor circuit, output meansto derive an amplified signal from said transistor circuit and aresistor-capacitor parallel bias network interposed in said circuit,said system comprising a control transistor having base, emitter andcollector electrodes, said control transistor being effectivelyconnected in series with said capacitor across said resistor, saidcollector electrode of. said control transistor being connected solelyto one end of said capacitor. rectifier and filter means coupled to saidoutput means to derive a direct control voltage therefrom proportionalto the volume level of the amplified signal and means to apply saidcontrol voltage to said control transistor to vary the impedance thereofaccordingly and thereby to control the amplification factor of saidamplifying transistor.

2. An automatic volume control system for a transistor amplifierprovided with an amplifying transistor having a base and emitter andcollector electrodes, input means to apply an input signal to said baserelative to one of said electrodes, output means to derive an amplifiedsig- 11:11 from the other of said electrodes relative to said oneelectrode, and a resistor-capacitor parallel network inter posed betweensaid one electrode and said input and output means to apply bias to saidone electrode, said system comprising a control transistor having baseemitter and collector electrodes, said control transistor beingeffectively connected in series with said capacitor across saidresistor, said collector electrode of said control transistor beingconnected solely to one end of said capacitor, means to derive a controlvoltage from said amplifier proportional to the volume level of theamplified signal therein, and means to apply said control voltage tosaid control transistor to vary the impedance thereof accordingly.

3. An automatic volume control system for a transistor amplifierprovided with an amplifying transistor having a base and emitter andcollector electrodes, input means to apply an input signal to said baserelative to one of said electrodes, output means to derive an amplifiedsignal from the other of said electrodes relative to said one electrode,and a resistor-capacitor parallel network interposed between said oneelectrode and said input and output means to apply bias to said oneelectrode, said system comprising a control transistor having base,emitter and collector electrodes, said control transistor beingeffectively connected in series with said capacitor across saidresistor, said collector electrode of said control transistor beingconnected solely to one end of said capacitor,

rectifier and filter means coupled to said output means to derive adirect control voltage therefrom proportional to the volume level of theamplified signal, and means to apply said control voltage to saidcontrol transistor to vary the impedance thereof accordingly and therebyto control the amplification factor of said amplifying transistor.

4. An automatic volume control system for a transistor amplifierincluding an amplifying transistor having a base, an emitter and acollector, input means to apply an input signal to said base relative tosaid emitter, output means to derive an amplified signal from saidcollector relative to said base, and a resistor-capacitor parallelnetwork interposed between said emitter and said input and output meansto apply bias to said emitter, said system comprising a controltransistor having base, emitter and collector electrodes, said controltransistor being efiectively connected in series with said capacitoracross said resistor, said collector electrode of said controltransistor being connected solely to one end of. said capacitor,rectifier and filter means coupled to said output means to derive adirect control voltage therefrom proportional to the volume level of theamplified signal and means to apply said control voltage to said controltransistor to vary the impedance thereof accordingly and thereby tocontrol the implification factor of said amplifying transistor.

5. A transistorized audio amplifier having an automatic-volume controlsystem comprising a first amplifying stage including a transistor havinga base, an emitter and a collector, means to apply an input signal tosaid base relative to ground, a resistor-capacitor parallel networkconnected between said emitter and to ground, an output amplifying stagecoupled to said collector to derive the amplified signal from said firststage, a control tran sistor interposed between said capacitor and saidresistor, the collector of said control transistor being connectedsolely to one end of said capacitor and the emitter thereof beingconnected to one end of said resistor, and means coupled to said outputstage to derive a direct control voltage therefrom proportional to thevolume level of the amplified signal and to apply said voltage to saidcontrol transistor to vary the impedance of said control transistoraccordingly,

6. A transistorized audio amplifier having an automaticvolume controlsystem comprising a first amplifying stage including a transistor havinga base, an emitter and a collector, means to apply an input signal tosaid base relative to ground, means to apply a negative potential tosaid collector relative to ground, a resistor-capacitor parallel networkconnected between said emitter and to ground, an output amplifying stagecoupled to said collector to derive the amplified signal from said firststage, a control transistor interposed between said capacitor and saidresistor the collector of said control transistor being connected solelyto one end of said capacitor and the emitter thereof being connected toone end of said resistor, and rectifier and filter means coupled to saidoutput stage to derive a direct control voltage therefrom proportionalto the volume level of the amplified signal and to apply said voltage tosaid control transistor to vary the impedance of said control transistoraccordingly.

7. In a circuit provided with a resistor-capacitor network, means tocontrol the reactance of said network comprising a control-transistorinterposed between said capacitor and said resistor, and said transistorhaving a collector connected solely to one end of said capacitor and anemitter connected to one end of said resistor, and means to apply acontrol voltage to the base of said transistor.

8. A gain controlling circuit for a transistor amplifier having gainvarying means, said circuit comprising variable impedance controltransistor means having, an input, a c llector, and an emitter-collectorcircuit, interconnection means interconnecting said emitter-collectorcircuit with said gain varying means of said amplifier, said inter!connecting means having means for limiting current flow in said cllector to the region of substantially zero direct current, and meansoperatively connected to said input for applying a control signal tosaid input of said control transistor means whereby the impedance ofsaid emitter-collector circuit thereof varies during operation in saidregion to thereby vary the gain of said amplifier.

9. A gain controlling circuit as defined in claim 8 in which saidtransistor amplifier includes a plurality of stages, one of said stagesincluding a transistor having said gain varying means coupled to anelectrode thereof.

10. A gain controlling circuit as defined in claim 9 in which said gainvarying means comprise a degenerative network which is controllablyby-passed by said variable impedance emitter-c llector circuit.

1]. A gain contr l circuit as defined in claim 10 in which said controlsignal is derived from the output of said transistor amplifier.

,12. An automatic volume control circuit for a transistor amplifierhaving gain varying means and an output, said control circuit comprisingvariable impedance control transist r means having an input, a cllector, and an emitter-collector circuit, means interconnecting saidemitter-collector circuit with said gain varying means of saidamplifier, said interconnecting means having an electricalcharacteristic for substantially eliminating direct current flow in saidcollector, and means interconnecting the output of said transistoramplifier with said input of said control transistor means whereby theemitter-collector impedance thereof varies as a function of saidamplifier output while said direct current flow is substantiallyeliminated, to thereby control the gain of said amplifier.

13. Control means for controlling the impedance of a network comprisingcontrol transist r means having a collector and an emitter-collectorcircuit connected to said network, means operatively connected to saidcontrol transistor means for controlling the flow of current in saidcollector to the zero direct current region and means operativelyconnected to said control transistor means for controlling the basecurrent of said transistor to control the conductance of saidemitter-collector circuit whereby the impedance of said network iscontrolled during operation in said region.

14. Contr l means for controlling the impedance of (1 network comprisingtransistor means having an emittercollector circuit connected to saidnetwork, means operatively connected to said emittencollector circuitfor forming a path with the collector of said transist r means beingsubstantially v id of direct current flow, and means operativelyconnected to said transistor means for controlling the base current forsaid transistor means to control the conductance of said emitter-cllector circuit whereby the impedance of said network is controlledwhile said collector is substantially void of direct current flow.

References Cited by the Examiner The following references, cited by theExaminer, are of record in the patented file of this patent or theoriginal patent.

UNlTED STATES PATENTS 2,307,308 1/1943 Sorensen. 2,323,634 7/1943 VanSlooten. 2,544,211 3/1951 Barton. 2,576,145 11/1951 Rudkin. 2,786,9643/1957 De Witt.

3,015,076 12/1961 Sheehan 330-29 X 3,023,369 2/1962 Horowitz 330145 XFOREIGN PATENTS 216,799 8/1958 Australia.

OTHER REFERENCES Shea: Principles of Transistor Circuits, 1953, page350.

ROY LAKE, Primary Examiner.

R. P. KANANEN, Assistant Examiner.

